Abnormal combustion or knock is disadvantageous in that it is audible to the driver at low engine speeds and reduces his perception of engine smoothness and also causes rapid destruction of the piston due to overheating if allowed to become excessive.
Knock can be reduced by retarding the ignition timing, enriching the fuel-air mixture or adjusting the exhaust gas recycling rate.
It is known to provide an accelerometer connected to the engine to listen for knock by sensing increased vibration.
Such a system is adequate at low engine speeds to reduce the noise audible to the driver but at higher engine speeds, when most damage is produced by knock, the ability of such accelerometer based systems to distinguish knock from the general increase in engine noise and vibration is greatly reduced.